Disconnect One Source Before Connecting the Other.
Open-transition transfer uses a break-before-make philosophy.
Conceptually:
SOURCE A CONNECTED
- Source A opens
- intentional interruption
- Source B closes
- load restored.
WHY OPEN TRANSITION IS COMMON
It creates clear electrical separation between the two sources during transfer.
That can simplify source interaction because they are not intentionally paralleled during the switching event.
WHAT THE LOAD EXPERIENCES
The load sees an interruption.
The duration depends on the system architecture and transfer sequence.
For many standby applications, this may be acceptable.
For others, UPS or other continuity technologies may bridge the interruption.
ENGINEERING QUESTIONS
Can the load tolerate interruption?
What equipment drops out?
What equipment restarts automatically?
What motors will restart?
What is the retransfer philosophy?
Does the UPS bridge the transfer?
LOAD RESTORATION
Even if transfer is successful, the returning load may create a substantial transient event.
Consider:
- compressors
- pumps
- HVAC
- lifts
- transformers
UPS recharge.
Open transition separates the sources—but the load must still tolerate the break.
Engineering intent
Open-Transition ATS should be evaluated as part of the complete power system rather than as an isolated catalogue item. Operating duty, connected load, interfaces, protection, controls, installation conditions, testing requirements and lifecycle consequences determine whether the arrangement is appropriate.
Control-system requirement
The control architecture must connect sensing, source readiness, permissives, breaker or transfer commands, feedback, alarms, communications and fallback modes. An individual component test does not prove the complete operating sequence.
Engineering considerations
The sources are not intentionally paralleled during the transfer. Open transition normally introduces a transfer interruption for loads not independently supported by stored energy. The acceptable duration depends on the load class. No-break loads should rely on the appropriate continuity architecture such as UPS rather than expecting an ordinary mechanical open-transition ATS to create no interruption by itself. Loads capable of tolerating transfer interruption can be connected through the appropriate emergency-transfer architecture. The controller should issue the required normal-source disconnect command and verify the actual device state. Do not proceed solely because the command was issued. If the normal source switching device does not open: This can provide a physical layer preventing simultaneous closure.
Selection and design considerations
The system should prevent unintended parallel connection unless the design specifically includes synchronised overlap. A transfer system must prevent dangerous backfeed into the unavailable or isolated source. Transfer architecture and interlocks must be designed to prevent unsafe energisation paths. Open transition transfer disconnects one source before connecting the other. Open transition is common in many standby applications, but its suitability depends on the load and project requirement. Delayed transition can intentionally hold the load disconnected between sources for a defined period. It may be used for certain load or source conditions. This must never be treated as a simple setting change. If overlap is required, synchronisation engineering becomes relevant. An ATS can transfer automatically and still allow an interruption.
Integration and protection considerations
“What interruption and power-quality conditions can this load tolerate?” life-safety systems • healthcare equipment • data/IT infrastructure • process controls • telecom • industrial processes • essential pumping • security systems. But an open-transition ATS introduces an interruption during transfer. The UPS supports the load during the source-transfer/start interval within its design. Required transfer performance, fault behaviour and control architecture must be verified. Then identify which protection function owns each fault.
Testing, commissioning and verification
inside the ATS controller • genset controller • synchronising/PMS controller • PLC. A conventional open-transition ATS normally does not intentionally parallel utility and DG. The study should define what constitutes an acceptable source. If utility voltage falls below the configured acceptable region for the required duration, the transfer sequence can begin.
Operating and lifecycle considerations
Bus-A can experience a transfer interruption unless another continuity layer supports its loads. An appropriately designed UPS may bridge no-break loads during open-transition restoration. Closing Main-A while the tie and Main-B remain closed can parallel Source-A and Source-B; this is permissible only in a system explicitly engineered for that condition. Retransfer and the return of shed loads must follow the approved operating sequence, including the required source checks and capacity assessment. Confirm the actual topology, interlocks and protection before applying this guidance.
Decision and application considerations
In practical installations, both functions may be incorporated into one control/switchgear package. AMF decides and coordinates. ATS switches the power path. When required conditions are satisfied, the load can be transferred according to the selected architecture. The transfer arrangement must prevent unsafe source interaction. In an open-transition arrangement, one source is disconnected before the other is connected. This avoids intentional paralleling of the two sources. Certain specialised architectures can allow controlled overlap or paralleling during transfer. Closed transition should never be represented as merely changing a software setting inside an ATS. Control and switchgear interlocking can prevent prohibited operating conditions. Examples can include preventing inappropriate simultaneous connection of: where the system is not designed for parallel operation.
Direct answer
Disconnect One Source Before Connecting the Other. Open-transition transfer uses a break-before-make philosophy.
The practical decision for Open-Transition ATS should be based on verified load, equipment, interface and site information. Where equipment-specific data, regulatory applicability or a project study is required, this guidance does not replace that verification.